Modified traveling wave tube



April 21, 1953 J. E. GORHAM MODIFIED TRAVELING WAVE TUBE Filed Oct. 2, 1950 INVENTOR.

JOHN E. GORHAM- Patented Apr. 21, 1953 T I MODIFIED TRAVELING WAVE TUBE John E. Gorham, Asbury Park, N. J., assignor to the United States of America as represented by the Secretary of the Army Application October 2, 1950, Serial No. 188,043

(Granted under Title 35, U. S. Code (1952),

see. 266) 2 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to electron discharge devices, particularly to such devices suitable for obtaining amplification at ultra-high frequencies.

in the conventional traveling wave tube employing a metallic helix as a radio frequency signal propagating circuit, amplification is achieved by the interaction between an electron beam and the electric component of the electromagnetic field established by the signal as it is propagated along said helix. The electron beam is shot through the axis of the helix, or along the outer surfaces thereof, in a direction parallel to the axis of the helix. An appropriate accelerating D. 0. potential is provided so that the velocity of the electron beam is made substantially equal to the phase velocity or" the input signal as it is propagated along the helix.

It is well known that as the frequency of the propagated signal, or traveling wave, is increased, the wave tends to cling close to the helix and consequently the electric intensity near the axis of the helix, where the electrons are, is greatly decreased. It can readily be seen that tubes of this type employing a metallic helix have a definite upper frequency limitation. Thus, traveling wave tube amplifiers of the conventional type described become ineifective, for all practical purposes, at extremely high frequencies.

It is an object of my invention, therefore, to provide an improved traveling wave tube amplifier which avoids the above-mentioned limitation.

It is another object of my invention to provide a traveling wave tube amplifier wherein the signal propagating circuit employed is made of appropriate low loss dielectric material.

It is still another object of my invention to provide a traveling wave tube employing a dielectric helix.

In accordance with my invention, I propose a modification of the conventional traveling wave tube amplifier in which a helix made of a low loss ceramic material is employed in coupling relationship with an electron beam.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing.

In Fig. 1 of the drawing there is shown an elevation, in part section, of a traveling wave tube embodying my invention. Fig. 2 shows a transverse section of Fig. 1 taken along line AA.

Referring now to the drawing, there is shown a traveling wave tube generally designated at 2 comprising an electron gun section 4 which includes cathode 6 and beam focussing anode 8, a dielectric helix I0, and collecting anode l2. It is to be understood that cathodes in combination with electric and/ or magnetic focussing devices for producing and directing a beam of electrons are old and that various particular forms of apparatus have been developed for this purpose, some of which may be substituted for the particular apparatus shown in the drawing.

Helix ill, which is made of quartz or any other suitable low-loss ceramic material, is conventionally positioned intermediate electron gun section 4 and collector anode 12, said helix being axially aligned with electron beam 14 which is projected from said electron gun. The supporting means of helix l0 inside the tube have been omitted from the drawing for the sake of simplicity but it will be understood that they may be of any convenient and suitable type, such as precision drawn low-loss glass tubing.

Projecting inwardly into the input and output ends of helix ID are tapered stubs l6 and I8 respectively, which in turn, project from coupling collars 2|] and 22. Said collars may be made of molybdenum or any other suitable material. Portions of waveguide, not shown, may be respectively coupled to collars 20 and 22 in the conventional manner to provide the radio frequency signal input and output circuits. Conventional transition waveguide sections and helix pitch variation may be provided to effectively match the dielectric helix to the waveguide input and output circuits. A D. C. potential may be applied between collar 22 and cathode B by battery 24 or any other suitable source. This potential is of such a magnitude that the velocity of the electron beam, as it passes through or along helix I0, is adjusted so that amplification of the externally applied radio frequency signal is achieved as it is propagated along said helix. As illustrated, a suitable source of positive potential may be applied to anode I2 in the usual manner. To prevent accumulation of charge and consequent interference with the electron beam, the surface of helix Ill may be coated as at 26 with carbon, platinum or any other suitable material to form a high resistivity coating. This semi-conductive metallic sheath permits leakage of electrons which may accumulate along the surface of the helix without appreciably affecting the leakage of electromagnetic flux.

In operation, the electron beam interacts with the electric component of the electromagnetic field established by the input radio frequency signal to achieve amplification of said signal as it is propagated along dielectric helix [0. Said electric component is assumed to be parallel to the axis of the helix and in coupling relationship to said electron beam. At very high frequencies, in tubes employing a conventional metallic helix, the radio frequency energy may be considered as traveling essentially in the air space between the metal turns. For a dielectric helix, however, the radio frequency energy, may be considered as essentially in the dielectric material, but with a, larger percentage coupling to the electron beam than in the case of the metal helix. As the electron beam is projected along the axis of helix H], or along the outer surfaces thereof, it can readily be seen that by employing a dielectric helix, a larger portion of the electromagnetic flux may be in coupling relationship with said beam than in the case of a metallic helix.

As hereinbefore mentioned, to prevent the accumulation of charges and consequent interference with the electron beam, the surface of the dielectric helix is coated with a semi-conductive layer 26 to permit leakage of any electrons which may impinge on the helix. It is well known that the variations in field leakage for dielectric waveguide conductors is a function of the ratio of wavelength to conductor radius. Thus, in order to compensate for reduction of fringing due to the partial shielding by the semi-conductive sheath, the ratio of wavelength to radius of the dielectric helix may be altered to a higher ratio value.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In an electron discharge device having at one end thereof means for projecting a beam of electrons over a predetermined path with a predetermined velocity and electron collector means at the other end thereof, means intermediate said beam projecting means and electron collector means for propagating an electromagnetic wave at approximately said velocity, said means comprising a dielectric helically formed wave colductor adjacent the path of said beam and in coupling relation thereto, said wave conductor having an input end and an output end, a first coupling collar at the input end of said dielectric wave conductor, a first tapered stub aflixed to said first collar and projecting inwardly into said input end, a second coupling collar at the output end of said Wave conductor, a second tapered stub aflixed to said second collar and projecting inwardly into said output end, and a metallic sheath of high resistivity enveloping the surface of said dielectric conductor to prevent electrons from accumulating thereon.

2. The device set forth in claim 1 wherein said dielectric wave conductor is arranged coaxially to the path of said electron beam.

JOHN E. GORHAM.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,064,469 Haeff Dec. 15, 1936 2,300,052 Lindenblad Oct. 27, 1942 2,304,540 Cassen Dec. 8, 1942 2,367,295 Llewellyn Jan. 16, 1945 2,516,944 Barnett Aug. 1, 1950 2,575,383 Field Nov. 20, 1951 FOREIGN PATENTS Number Country Date 508,354 Great Britain June 29, 1939 

